Multiple collimator muffler

ABSTRACT

MUFFLER FOR PNEUMATIC EXHAUST, PARTICULARLY EXHAUSTS AT SUPERSONIC VELOCITIES. MUFFLER COMPRISES A PRIMARY EXPANSION CHAMBER AND SECONDARY SILENCING CHAMBER, AIR FROM THE PRIMARY CHAMBER PASSES THROUGH A PLURALITY OF COLLIMATING NOZZLES INTO THE SECONDARY CHAMBER, FROM WHICH THE AIR IS DISSEMINATED THROUGH A POROUS WALL OF ACOUSTICAL MATERIAL. THE NOZZLES AND THE STRUCTURE SUPPORTIN THEM APPEAR TO BREAK UP ULTRASONIC SHOCK WAVES AND DISSIPATE SOME OF THEIR ENERGY IN THE PRIMARY CHAMBER. INTERFERENCE BETWEEN JETS OF AIR ENTERING THE SECONDARY CHAMBER THROUGH THE NOZZLES ADDITIONALLY DISSIPATES SOUND ENERGY. FURTHER DISSIPATION IS OBTAINED BY IMPINGEMENT OF THE JETS AGAINST AND PASSAGE THROUGH ACOUSTICAL MATERIAL CONSTITUTING INTERNAL WALL SURFACES OF THE SECONDARY CHAMBER.   D R A W I N G

United States Patent Office 3,7l5,l Patented Feb. 6, 1973 3,715,010 MULTIPLE COLLIMATGR MUFFLER Stephen J. Gibel, 5846 Edgerton Road, North Royalton, Ohio 44133 Filed Feb. 2, 1972, Ser. No. 222,876 Int. Cl. F01n 1/10 U.S. Cl. 181-50 8 Claims ABSTRACT 0F THE DISCLOSURE Muler for pneumatic exhausts, particularly exhausts at supersonic velocities. Muffler comprises a primary expansion chamber and secondary silencing chamber; air from the primary chamber passes through a plurality of collimating nozzles into the secondary chamber, from which the air is disseminated through a porous wall of acoustical material. The nozzles and the structure supporting them appear to break up ultrasonic shock waves and dissipate some of their energy in the primary chamber. Interference between jets of air entering the secondary chamber through the nozzles additionally dissipates sound energy. Further dissipation is obtained by impingement of the jets against and passage through acoustical material constituting internal wall surfaces of the secondary chamber.

This invention relates to improvements in mulers for pneumatic exhausts, and, more particularly, to a high performance muffler for the exhausts emitted at high velocities with attendant high noise levels.

The silencing of the noise of high velocity exhausts of pneumatically operated tools and equipment presents a problem which is accentuated in the case of intermittently operated fast-acting devices such as pneumatic valves, brakes, clutches, and the like. The problem is also presented by high-speed pneumatic motors, drills, hammers, and the like which may be run relatively constantly, but which exhaust their operating air at a high velocity that is, in fact, the average of short but much higher velocity bursts at a high frequency.

The problem is the somewhat paradoxical one of providing a muler of a size that is small enough, both economically as to its cost and which will also meet the space limitation at the site of use, on the one hand, and, on the other, which will dissipate the large volume and kinetic energy of the high velocity exhausted air without creating back pressure that may cause malfunction of the pneumatic device or, at least, seriously impair its eiciency. Because the ver'y design and function of such a device requires that the operating air be exhausted at a high velocity and without substantial restriction in the muier for its exhaust, the art has simply accepted, as a necessary evil, mutilers which, so long as they do not impair the efiiciency of the devices, reduce the exhaust noises to any level less than that of the unmuflled exhausts; the actual discomfort often caused by the still high noise level was regarded as a factor that had to be tolerated. One excuse for the relative inelfectiveness, from a noisereduction standpoint, of prior art mulers for high velocity exhausts has been that the air may be exhausted at momentary ultrasonic velocities. The resultant shock waves were assumed to be inherently impossible to suppress without creating restrictions and back pressures that could disactivate or impair the function of the pneumatic devices.

It is an object of this invention to provide a high performance mutller for pneumatic exhausts which `can muffie to substantially lowered noise levels such high velocity exhausts, including exhausts emitted at supersonic velocities, without creating intolerable back-pressure and restriction in the flow of exhaust air. Another object of this invention is to provide a muflier which is relatively compact in comparison with the size mufflers heretofore available that were able to effect a corresponding reduction in the noise levels of high velocity pneumatic exhausts. The increased cost of manufacture of mufilers made according to this invention is frequently more than offset by the higher degree of noise attenuation which can be obtained without a corresponding greater restriction of air flow from a pneumatic exhaust.

Other objects and advantages of this invention will be apparent from the following specication, claims, and the accompanying drawings, in which:

FIG. 1 is an end view of a mufller made according to this invention.

FIG. 2 is a side view, partly in section, of the muffler shown in FIG. l.

FIG. 3 is a cross-section taken along the line 3-3 of FIG. 2 and showing the nozzle plate used in the illustrated embodiment of this invention.

FIG. 4 is an enlarged detail section taken along the line 4 4 of FIG. 3.

As shown in the drawings illustrating a mutiler made according to this invention, the embodiment shown is comprised of a front cover 10 and a back cover 30, each provided with outer internal flanges 11 and 31, respectively. A tubular body 20 is held in these flanges by a pair of tie rods 21 extending through the covers 10` and 30 and the secondary silencing chamber 25 defined by the body 20; the tie rods are tensioned by suitable nuts 22 bearing against the outer surfaces of the covers 10` and 30 whereby the muier may be disassembled to allow the porous acoustical material lining the secondary silencing chamber to be replaced if it becomes clogged during use with oils and moisture entrained in the exhaust air.

The domed front cover 10 is provided with an outer central nut-faced boss 12 having a threaded bore, the boss 1.2 thereby constituting a fitting by which the mutlier may be attached to the exhaust pipe of pneumatic devices to be silenced or connected by a suitable nipple (not shown) to the exhaust port of such a device. Except for internal bosses 13 drilled to receive the tie rods 21 and the llet of the side wall of the dome connecting its roof to the seat of the flange 11, the interior of the dome of the cover 10l provides a substantially unobstructed cylindrical primary expansion chamber 14 for the mufller. AS a consequence, its proportion and dimensions are relative to the inlet bore of the pipe or fitting through which the exhaust air enters the chamber 14. It has been determined empirically that the axial depth of the chamber should be in the order of 1A to V2 of the diameter of the inlet bore; a lesser depth may unduly constrict the expansion of air from the inlet bore into the chamber and the expansion permitted by a greater depth does not appreciably enhance the performance of the muffler but may unnecessarily increase its cost and size. It has also been found empirically that the proportion of the axial depth of the chamber 14 to its diameter should be in the order of 1:3.5 and preferably in the order of 1:5; if the desired proportion of the axial depth of the chamber to the inlet bore is observed, an increase in the optimum proportion of the diameter of the chamber relative to its depth will not provide a corresponding increase in the performance of the muffler warranted by the increased size and cost of it.

The back cover 30- is closed and thus omits the boss 12 of the front cover 10; for symmetry the configuration of the back cover 30 is otherwise similar to that of the front cover 10, but its proportions are not significant with respect to the function of the muflier. To provide support for the disk of porous acoustical material 35 constituting an end Wall of the secondary silencing chamber of the body 20, the back cover maybe provided with an internal central boss 32 equal in axial depth to that of the seat of the flange 31 and the internal bosses 33` in which the tie rods 22 are received.

The body 20 comprises the internal tubular wall 24 of the secondary silencing chamber 25; it is a porous acoustical material, preferably loosely fitted cellulosic fibers bonded with resinous binder to provide necessary mechanical strength. The compression load imposed on the body 2li by the tie rods 2-1 is largely carried by an outer shell 26 of relatively thin perforated metal or screening, which also serves to protect the inner wall 24 from abrasion and damage from impacts during use. The acoustical material of the inner wall 24 provides a great multiplicity of sound-absorbing tortuous passages from the chamber 25 to the surrounding atmosphere. The open portions of the shell 26 should be substantially equally spaced in the surface area of the shell 26 and their total area should be in the order of 40% or more of its total surface area in order to not unduly restrict the escape of exhaust air through the sound-absorbing wall 24. The internal area of the tubular Wall 24 and its thickness and porosity should be such that, together with the shell 26, there should be no significant increase in the air pressure at the inner surfaces of the wall 24 over that of the ambient atmosphere into which the exhaust air escapes through the openings in the shell 26.

Except for the use of a domed front cover corresponding to a front cover as disclosed in my copending application Ser. No. 175,175, filed Aug. 26, 1971, for an Expansioned Chambered, Fail-safe Muffler and the employment of disk of sound-absorbing acoustical material 315 lining the end wall of the secondary chamber 25, the construction of the muffler as described thus far is substantially conventional. Where the construction of a muier made according to this invention differs radically from prior art mufllers of otherwise general similar construction is in the provision of a nozzle plate separating the primary expansion chamber F14 from the secondary silencing chamber 25,. Whereas the prior art devices employed baffles, screening, or thin sheets of highly perforated metal similar to that used in the shell 26 in order to allow air to enter the secondary silencing chamber with an apparent minimal restriction from its initial inlet into the mufiler provided by the means for connecting the muler to the exhaust pipe or port of a pneumatic device, the relatively heavy nozzle plate 15 would entirely close olf the primary expansion chamber from the secondary silencing chamber but for the presence of a relatively limited number of tapered nozzle shaped passageways extending through it. l have discovered that if the plurality of nozzle passageways 16 (hereafter nozzles) are of sucient length to cause the exhaust air introduced into the primary chamber 14 to enter the chamber v25 as axially collimated jets traversing the essentially radial llow of air through the Wall 24, then, provided the total exit area of the nozzles is not substantially less than the area of the inlet bore into the primary chamber 14, not only does such a plate offer no significant restriction to the flow of air into the secondary chamber, but a substantially greater noise reduction is obtained with such a muller as a whole. This improved silencing is particularly evident if the exhaust enters the mufiier at high velocities which may approach or exceed supersonic velocities.

The nozzles 16 are located in the plate so that they will be in the apparent envelope of the shock wave of high velocity air entering the primary chamber; they are preferably arranged in a minimum of three of more equally spaced groups of relatively closely adjacent nozzles, also equally spaced from each other. The number of such nozzle-groups and the number of nozzles in a group may be increased, but empirically and within practical limits of the cost of drilling such nozzles, no discernible irnprovement appears to be obtained if the total exit area of all nozzles exceeds twice the area of the inlet bore or if the number of groups exceeds seven to nine or the number of nozzles in a group exceeds that number.

The plurality of nozzles 16 may be formed in a single solid plate by tapered drills, but are ordinarily most economically formed by providing a rigid plate 1S with inserted plugs 17. The plugs 17, having a diameter substantially equal to the bore of the inlet to the chamber 14, are conveniently of the type disclosed in my Pat. No. 3,537,543, issued Nov. 3, 1970, for Noise Muiiied Air Ejector and my copending application, Ser. No. 162,784, iiled July 15, 1971, for Self-Pressure Regulating Air Ejector, each plug 17 thereby providing the desirable grouping of nozzles. Performance is improved if, as shown in FIG. 4, the yfrontal nozzle entrance surface of the plug is likewise dished to a depth at least approximately equal to the exit area of a nozzle passageway 16, the entrance area of each nozzle being thereby enlarged while maintaining its outer edge substantially tangential to the circumference of the plug. Thus, in a plug having the proportions disclosed in my aforesaid patent, the diameter of the locus of the axes of the nozzles in a group is approximately equal to the diameter of the bore of the chamber inlet less twice the diameter of the exit of a nozzle. The taper of the nozzle is determined by the number of nozzles in a plug and its length. The length of a plug, as empirically determined, should be at least a third of its diameter and may exceed its diameter in length, though no improvement in performance has been noted in plugs whose lengths are substantially greater than their diameters.

Mulers made according to this invention effect substantial noise attenuation without creating undesirable back pressures interfering with the operation of the muled pneumatic devices. For example in a test made upon a pneumatically actuated valve from which the operating air was exhausted through a '1/2" pipe-size port at p.s.i., no interference `with the timing of the valve operation was eifected by a muier of the above desirable construction. A noise attenuation of 33 decibels, however, was attained, that is, a sound pressure level of the unmuflled exhaust was reduced from 117 decibels to 84 decibels; the best obtainable attenuation with a conventional prior art muffler which also allowed an air ow that did not interfere with the function of the pneumatic device was a reduction to 92 decibels, a barely tolerable level.

Why mufers made according to this invention are able to provide superior noise level attenuations without creating disabling interference in the air ow from the muled devices is not fully understood. It would appear, however, that a shock wave emanating from the inlet to the primary chamber is broken up as it strikes the plate 15 and its dished portions provided by the frontal areas of the plugs 17 and some of its energy is dissipated by interference with standing portions of the diversely reliected Wave. The absence of a significant static back-pressure and restriction of air liow into, the mufer indicates that the effect of the nozzles may be to accelerate the velocity of the air ilow in each individual nozzle to a supersonic velocity or nearly so. And while the ow of air within each )et may be turbulent and it would be expected to regenerate a shock wave as it leaves a nozzle exit, the overall effect of the jets in a grouping is that of a high velocity jet stream having laminar flow and in which the noise level is reduced by interference between adjacent jets and the cancellation of sound and/or shock waves which each jet may create. Further dissipation of sound energy may be accomplished by interference between the adjacent jet streams in the system, the spreading of the jet streams as they impinge -upon the porous acoustical material lining the silencing chamber and as the general axial direction of travel of the jet streams within the silencing chamber traverses the essentially radial component of the ow of air through the side walls 24. The final dissipation of sound energy, of course, presumably takes place within the porous wall 24 in the conventional manner.

It is to be understood that the term air as used in the foregoing specification and appended claims includes other gases and vapors. It is also to be understood that this invention is not to be deemed limited to the specific embodiment described above and shown in the drawings; rather, this invention may be varied and modified within the limits of the appended claims without departing from the spirit and scope thereof.

What is claimed is:

1. In a mufller for the exhaust of a pneumatically operated device, a closed casing comprised of a primary expansion chamber into which the exhaust is introduced through thebore of an inlet to said chamber and an axially aligned secondary silencing chamber of substantially larger volume than said primary chamber into which air flows from the primary chamber, said secondary chamber having porous Walls through which air may escape to the ambient atmosphere, and a system of a plurality of nozzles by which air escapes from the said primary chamber into said secondary chamber as a plurality of collimated jets, said jets being configured to increase the velocity of air discharged from their exits with respect to the velocity at which said air enters them.

2. A muler as defined in claim 1 in which the total area of the exits of said nozzles is not substantially less than the bore area of said inlet.

3. A mutlier as defined in claim 2 including a transverse wall separating said primary and secondary charnbers and in which the plurality of nozzles extend axially through said wall and are grouped in not less than three groups, each group being located so as to be equally spaced from a pair of adjacent groups and each group is comprised of at least three nozzles equally spaced from each other, whereby air from the primary chamber enters said secondary chamber in axially extending jet streams comprised of collimated jets.

4. A muffler as defined in claim 3 in which said Wall includes a plurality of plugs inserted therethrough, each plug being drilled axially to provide the nozzles of a group.

5. A muliier as defined in claim 4 in which said plug is substantially equal in diameter to the diameter of the bore of the inlet to said primary chamber, the nozzles in each plug being tapered from a larger entrance opening to a smaller exit opening and the outer edges of the entrance openings of nozzles in the group are located adjacent to the circumference of their plug.

6. A muler as defined in claim 5 in which the length of each plug is at least substantially one-third of its diameter and the entrance surface of each plug is dished to a depth at least substantially equal to the diameter of the exit of a nozzle extending therethrough.

7. A muier as defined in claim 6 in which the primary expansion chamber provides a substantially cylindrical volume into the center of which the bore of said inlet opens, the axial depth of said cylindrical volume being at least one fourth the diameter of said bore and the diameter of said cylindrical volume being at least three times the diameter of said bore.

8. A muffler as defined in claim 7 in which the volume of the said secondary chamber is substantially cylindrical and its inside diameter is substantially equal to the diameter of said primary expansion chamber, the axial length of said secondary chamber being in the order of six or more times the axial depth of said primary chamber, and an end Wall of acoustical sound absorbing material closing the end of said secondary chamber opposite the wall containing nozzles through which air enters said secondary chamber.

References Cited UNITED STATES PATENTS 1,739,039 12/1929 Powell 18l-35 R 2,600,236 6/1952, Gibel 181-56 2,815,088 12/1957 Gibel 181-36 A UX 3,339,668 9/1967 Trainor 181-60 X 3,380,553 4/1968 Gibel l8l-60 X 3,537,543 11/1970 Gibel 181-60 X RIOHARD B. WILKINSON, Primary Examiner J. F. GONZALES, Assistant Examiner U.S. Cl. X.R. 181-36 A, 56 

